An herbicide that contaminates the tap water consumed by millions of Americans has been found to produce gender-bending effects in male frogs, "chemically castrating" some and turning others into females, a study shows.

Frogs in the experiment were exposed to amounts of the weedkiller atrazine that are comparable to the levels allowed in drinking water by the Environmental Protection Agency, says lead researcher Tyrone Hayes of the University of California-Berkeley.

The study was released Monday in the Proceedings of the National Academy of Sciences.

In Hayes' earlier studies, atrazine caused male frogs to begin growing eggs in their testes. In this experiment, 10% of the males exposed to atrazine — one of the most commonly used herbicide in the world — actually changed sex; some were able to breed and lay eggs. Nearly all of the other males had low testosterone and sperm levels, which made them unable to reproduce, Hayes says.

The experiment can't tell scientists whether atrazine affects humans in a similar way, Hayes says. But it does raise new questions about the safety of atrazine, which other studies have linked to human birth defects, low birth weight, prematurity and low sperm count.

The EPA said in October that it planned to reassess atrazine's safety, including its cancer risk. The European Union has banned the chemical.

About 75% of stream water samples and 40% of groundwater samples contain atrazine, according to the U.S. Geological Survey. The Natural Resources Defense Council, an environmental advocacy group, detected atrazine in 90% of tap water samples from 139 water systems. Inexpensive faucet-top water filters can remove the chemical, says Jennifer Sass, a scientist with the group.

Several water systems in Illinois, where atrazine is commonly used on corn and other crops, are suing the chemical's manufacturers, asking them to pay the cost of taking the herbicide out of drinking water.